Coated optical fiber and methods of making

ABSTRACT

A drawn optical fiber (21) is provided with inner and outer layers (42,44) of coating material to protect the optical fiber during handling and use. The coating materials are such that they are characterized by being curable upon exposure to different portions of the light spectrum. In a preferred embodiment, the coating material of the inner layer includes a photoinitiator which absorbs light in the visible portion of the light spectrum whereas the outer coating material of the outer layer includes a photoinitiator which absorbs light in the ultraviolet light portion of the light spectrum.

TECHNICAL FIELD

This invention relates to coated optical fiber and to methods of makingsame. More particularly, the invention relates to an optical fiberhaving inner and outer layers of curable coating materials, one of whichmay be curable by exposure to the visible light spectrum.

BACKGROUND OF THE INVENTION

In the manufacture of optical fiber, a glass preform rod which generallyis manufactured in a separate process is suspended vertically and movedinto a furnance at a controlled rate. The preform softens in the furnaceand optical fiber is drawn freely from the molten end of the preform rodby a capstan located at the base of a draw tower.

Because the surface of the optical fiber is very susceptible to damagecaused by abrasion, it becomes necessary to coat the optical fiber,after it is drawn, before it comes into contact with any surface.Inasumch as the application of a coating material must not damage theglass surface, the coating material is applied in a liquid state. Onceapplied, the coating material must become solidified rapidly before theoptical fiber reaches the capstan. This may be accomplished byphotocuring, for example.

Those optical fiber performance properties which are affected most bythe coating material are strength and transmission loss. Coating defectswhich may expose the optical fiber to subsequent damage arise primarilyfrom improper application of the coating material. Defects such as largebubbles or voids, non-concentric coatings with unacceptably thinregions, or intermittent coatings must be prevented. The problem ofbubbles in the coating material has been overcome. See, for example,U.S. Pat. No. 4,851,165 which issued on Jul. 25, 1989 in the names of J.A. Rennell, Jr. and C. R. Taylor. Intermittent coating is overcome byinsuring that the fiber is suitably cool at its point of entry into thecoating applicator to avoid coating flow instabilities. Coatingconcentricity can be monitored and adjustments made to maintain anacceptable value.

Optical fibers are susceptible to a transmission loss mechanism known asmicrobending. Because the fibers are thin and flexible, they are readilybent when subjected to mechanical stresses, such as those encounteredduring placement in a cable or when the cabled fiber is exposed tovarying temperature environments or mechanical handling. If the stressesplaced on the fiber result in a random bending distortion of the fiberaxis with periodic components in the millimeter range, light rays, ormodes, propagating in the fiber may escape from the core. These losses,termed microbending losses, may be very large, often many times theintrinsic loss of the fiber itself. The optical fiber must be isolatedfrom stresses which cause microbending. The properties of the opticalfiber coating material play a major role in providing this isolating,with coating geometry, modulus and thermal expansion coefficient beingthe most important factors.

Typically two layers of coating materials are applied to the drawnoptical fiber. Furthermore, two different kinds of coating materials areused commonly. An inner layer which is referred to as primary coatingmaterial is applied to be contiguous to the optical glass fiber. Anouter layer which is referred to as a secondary coating material isapplied to cover the primary coating material. Usually, the secondarycoating material has a relatively high modulus, e.g. 10⁹ Pa, whereas theprimary coating material has a relatively low modulus such as, forexample, 10⁶ Pa. In one arrangement, the primary and the secondarycoating materials are applied simultaneously. Such an arrangement isdisclosed in U.S. Pat. No. 4,474,830 which issued on Oct. 2, 1984 in thename of C. R. Taylor.

Subsequently, both the inner and the outer layers of coating materialsare cured beginning from the outside progressing inwardly. Alsotypically, the primary and the secondary coating materials compriseultraviolet light curable materials each being characterized by aphotoactive region. A photoactive region is that region of the lightspectrum which upon the absorption of curing light causes the coatingmaterial to change from a liquid material to a solid material. Both thematerials which have been used for the primary and for the secondarymaterials have comparable photoactive regions. Because the photoactiveregions are comparable, the curing light for the primary coatingmaterial will be attenuated by the secondary coating material. As aresult of the attenuation, less light reaches the primary coatingmaterial.

Of course, notwithstanding the attenuation of the curing light by thesecondary coating material, it is important that the primary coatingmaterial be fully cured. This problem has been overcome in the prior artby reducing the line speed to allow longer exposure time of the primarycoating material to the ultraviolet curing light energy inasmuch as theultraviolet curing light energy is inversely proportional to line speed.

Although the foregoing solution is a workable one, it has itsshortcomings. Most importantly, any reduction in line speed is notdesirable and runs counter to current efforts to increase draw lengthsand to increase substantially draw speeds of the optical fiber.

What is needed and seemingly what is not disclosed in the prior art is acoated optical fiber which overcomes the foregoing problem ofattenuation by the secondary coating material of the light energy usedto cure the primary coating material. Any solution should be one whichdoes not affect adversely the line speed. Further, methods which must beimplemented to make such a sought after coated optical fiber must becapable of being integrated with present manufacturing arrangements fordrawing optical fiber from a preform.

SUMMARY OF THE INVENTION

The foregoing problems of the prior art have been solved by the coatedoptical fiber and methods of making same of this invention. A coatedoptical fiber of this invention includes optical glass fiber and aninner coating material which engages and which encloses the opticalglass fiber. The inner coating material is enclosed by an outer coatingmaterial which engages the inner coating material. The inner and theouter coating materials are such that they are characterized by beingcurable at different regions of the light spectrum. For example, theinner coating material may be on which is characterized as being curableupon exposure to the visible light spectrum and the outer coatingmaterial may be one which is characterized as being curable uponexposure to the ultraviolet light spectrum.

In a method of this invention, optical fiber is drawn from a preform.Then a primary and a secondary coating material are appliedsimultaneously to the drawn fiber, the primary and the secondary coatingmaterials being such that they are cured by exposure to differentportions of the light spectrum. In a preferred embodiment, the primarycoating material which is contiguous to the optical fiber is one thephotoactive region of which is in the visible light spectrum. On theother hand, the secondary coating material is one of which thephotoactive region is in the ultraviolet light spectrum. In thepreferred embodiment, the coating materials are cured first by exposingthe drawn coated optical fiber to a curing lamp which is characterizedby an emission spectrum exclusively in the visible light reigon.Subsequently, the secondary coating material is cured by exposing thedrawn coated optical fiber to a curing lamp characterized by an emissionspectrum exclusively in the ultraviolet light spectrum. The then drawn,coated optical fiber is taken up.

BRIEF DESCRIPTION OF THE DRAWING

Other features of the present invention will be more readily understoodfrom the following detailed description of specific embodiments thereofwhen read in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view of a manufacturing line for drawing opticalfiber from a preform;

FIG. 2 is an end view in section of a drawn coated optical fiber;

FIG. 3 is a graph which depicts a plot of absorbance verus wavelength inand in the vicinity of the visible light region;

FIG. 4 is a histogram which shows the emission output of a commerciallyavailable light curing bulb which has substantially most of its outputin the visible light region; and

FIG. 5 is a graph depicting modulus versus dose for a coating materialwhich is cured upon exposure to the visible light spectrum.

DETAILED DESCRIPTION

Referring now to FIG. 1, there is shown an apparatus which is designatedgenerally by the numeral 20 and in which is used to draw optical fiber21 from a specially prepared cylindrical preform 22 and for then coatingthe drawn fiber. The optical fiber 21 is formed by locally andsymmetrically heating the preform 22, typically 7 to 25 mm in diameterand 60 cm in length, to a temperature of about 2000° C. As the preformis fed into and through a furnace 23, fiber 21 is drawn from the moltenmaterial.

As can be seen in FIG. 1, the elements of the draw system include thefurnace 23 wherein the preform is drawn down to the fiber size afterwhich the filter 21 is pulled from a heat zone therein. The diameter ofthe fiber 21 is measured by a device 24 at a point shortly after thefiber is formed and this measured value becomes an input into a controlsystem. Within the control system, the measured diameter is compared tothe desired value and an output signal is generated to adjust the drawspeed such that the fiber diameter approaches the desired value.

After the diameter of the optical fiber 21 is measured, a protectivecoating system 25 (see also FIG. 2) is applied to the fiber by anapparatus 27. Preservation of fiber strength requires the application ofthe protective coating, which shields newly drawn fiber from thedeleterious effects of the atmosphere. This coating system must beapplied in a manner that does not damage the surface of the fiber 21 andsuch that the fiber has a predetermined diameter and is protected from aabrasion during subsequent manufacturing operations, installation andservice. Minimizing attenuation requires the selection of a suitablecoating material and a controlled application of it to the fiber. Such acoating apparatus may be one such as that described in priorlyidentified U.S. Pat. No. 4,474,830. Minimizing diameter variation whichin turn minimizes the losses due to misalignment at connector and splicepoints requires careful design of the draw system and the continuousmonitoring and control of the fiber diameter during the drawing and thecoating steps of the process. Then, the coated fiber 21 is passedthrough a centering gauge 28.

After the coating materials have been applied to the drawn fiber, thecoating materials must be cured. Accordingly, the optical fiber havingthe coating materials thereon is passed through a device 30 for curingthe coating system and a device 32 for measuring the outer diameter ofthe coated fiber. Afterwards, it is moved through a capstan 34 and isspooled for testing and storage prior to subsequent cable operations.

In the apparatus 27, the coating system 25 comprising two coatingmaterials are applied to the optical fiber. The coating system 25includes an inner layer 42 (see FIG. 2) which often is referred to as aprimary coating layer and an outer layer 44 which often is referred toas a secondary coating material. The coating material of the inner layerwhich has a substantially lower modulus than that of the outer layer, issuch that it prevents microbending of the optical glass fiber. On theother hand, the higher modulus outer layer provides mechanicalprotection for the drawn glass fiber.

Each of the coating materials is curable by being exposed to a portionof the light spectrum. Generally each of the coating materials includesan oligomer, a diluent and a photoinitiator. Also included may beadditives such as, for example, antioxidants, adhesion promoters,ultraviolet (UV) light stabilizers, surfactants and shelf lifestabilzers.

Importantly, the coating material of the inner layer 42 is such that itcures upon exposure to a different portion of the light spectrum thandoes the outer layer 44. As a result, the light energy which passesthrough the outer layer 44 and impinges on the inner layer 42 to curethe coating material thereof is not attenuated by absorption in theouter layer.

In a preferred embodiment, one of the layers of the coating system 25 iscurable upon exposure to the visible light spectrum and the other uponexposure to the ultraviolet light spectrum. More particularly, in thepreferred embodiment, the coating material of the inner layer 42 iscurable upon exposure to the visible light spectrum whereas the coatingmaterial of the outer layer 44 is curable upon exposure to theultraviolet light spectrum. To this end, the composition of the coatingmaterial of the inner layer 42 includes a photoinitiator which maycomprise camphorquinone. For the outer layer, the photoinitiator may bea 2,2 dimethoxy, 2 phenylacetophenone such as Irgacure 651 which ismarketed by the Ciba Giegy Company. The photoinitiator of the outerlayer which is ultraviolet light curable also may be a 1 phenyl, 2hydroxy, 2 methylpropanone such as Darocure 1173 which is marketed bythe EM Industries Company.

Going now to FIG. 3, there is shown a graph which depicts absorbance ofa coating composition of matter versus wavelength. The wavelengthsdepicted are those generally in what is considered to be the visiblelight spectrum. FIG. 3 shows that the coating composition of matter ofthe inner layer 42 absorbs in the visible light spectrum. If the coatingmaterial absorbs in a specified wavelength region, and inquiry must bemade as to how to radiate it in the region where absorbed. In FIG. 4there is shown a histogram of output in watts per inch for acommerically available bulb versus wavelength in nanometers.

When using a bulb having the output spectrum shown in FIG. 4 toirradiate the coating formulating having an absorbance plot as shown inFIG. 3, then the modulus follows the curve shown in FIG. 5. As shown inFIG. 5, when the proper lamp is used to irradiate the composition ofmatter that absorbs at the wavelengths of FIG. 4, then the inner layerwill cure to have moduli corresponding to the doses disclosed in FIG. 5.

Advantageously, the methods and the article of this invention allow theuse of higher cure speeds than used priorly. Because the curing energythat is used to cure the inner layer 42 is not attenuated by the outerlayer 44, not as much time is needed for exposure to overcome suchattenuation.

It is to be understood that the above-described arrangements are simplyillustrative of the invention. Other arrangements may be devised bythose skilled in the art which will embody the principles of theinvention and fall within the spirit and scope thereof.

I claim:
 1. A coated optical fiber, comprising:an optical glass fiber;an inner coating material which engages and encloses the optical glassfiber; and an outer coating material which engages and which enclosesthe inner coating material, the coating materials being ones which arecharacterized as being curable upon exposure to different portions ofthe light spectrum in that one of coating materials absorbs light energyin the visible light spectrum and the other in the ultraviolet lightspectrum.
 2. The coated optical fiber of claim 1, wherein said innercoating material includes a photoinitiator which absorbs in theultraviolet light region of the spectrum and the outer coating materiala photoinitiator which absorbs in the visible light spectrum.
 3. Acoated optical fiber, comprising:and optical glass fiber; an inner layercomprising a coating material which engages and encloses the opticalglass fiber, said coating material of said inner layer being one whichis characterized as being curable upon exposure to be visible lightspectrum; and an outer layer comprising a coating material which engagesand which encloses the inner layer, said coating material of said outerlayer being one which is characterized as being curable upon exposure oflight of the ultraviolet light spectrum.
 4. A coated optical fiber,comprising:an optical glass fiber; an inner layer comprising a curedcoating material which engages and encloses said optical glass fiber,said coating material of said inner layer being one which ischaracterized as having been cured upon exposure to light energy of thevisible light spectrum; and an outer layer comprising a cured coatingmaterial which engages and which encloses said inner layer, said coatingmaterial of said outer layer being one which is characterized as havingbeen cured upon exposure to light energy of the ultraviolet lightspectrum.
 5. The coated optical fiber of claim 4, wherein the coatingmaterials of said inner and outer layers are cured simultaneously. 6.The coating optical fiber of claim 4, wherein said inner coatingmaterial includes a photoinitiator which absorbs in the visible lightportion of the spectrum; andwherein said outer coating material includesa photoinitiator which absorbs in the ultraviolet light region of thespectrum.
 7. The coated optical fiber of claim 6, wherein saidphotoinitiator of said coating material comprises a camphorquinone. 8.The coated optical fiber of claim 6, wherein said photoinitiator of saidouter coating material comprises a 2,2 dimethoxy, 2 phenylacetophenone.9. The coated optical fiber of claim 6, wherein said photoinitiator ofsaid outer coating material comprises 1 phenyl, 2 hydroxy, 2methylpropanone.